Some vehicles used as a construction machine such as a wheel loader and a bulldozer are provided with hydraulic drive apparatuses called HST (Hydro-Static Transmission) between driving wheels and engines which are driving sources. The hydraulic drive apparatus includes a variable displacement hydraulic pump driven by the engine, and a variable displacement hydraulic motor which is driven by pressure oil discharged from the hydraulic pump. A driving force of the hydraulic motor is transmitted to a driving wheel to run the vehicle.
According to a vehicle to which the hydraulic drive apparatus is applied, by appropriately adjusting a capacity of the hydraulic pump and a capacity of the hydraulic motor, a ratio of the rotation number of the hydraulic pump and the rotation number of the hydraulic motor can freely be changed. Therefore, the speed of the vehicle can be changed continuously only by operation of an acceleration pedal without carrying out complicated lever operation, and the operability can be enhanced remarkably.
A general hydraulic motor has such characteristics that output torque becomes small if the hydraulic motor is rotated at high speed but the output torque is increased when the hydraulic motor is rotated at low speed. This relation between the rotation number and the output torque of the hydraulic motor is preferable when used on a normal road surface, because when a vehicle to which the hydraulic motor is applied runs at low speed, high output torque is transmitted to the driving wheel and the acceleration performance at the time of start of the vehicle is excellent.
However, the construction machine such as the wheel loader is frequently used on a low friction road surface such as a soft road surface and a snow road surface. In such a condition, the high output torque at the time of start of the vehicle promotes slip, and there is a possibility that it becomes difficult to obtain stable running performance.
To solve the above problem, in some of conventional hydraulic drive apparatuses, the upper limit of the output torque is set by electronically controlling the maximum capacity of the hydraulic motor. More specifically, the conventional hydraulic drive apparatus is constituted such that the maximum tilting angle can be changed using an colinoaxis type variable displacement hydraulic motor. According to this hydraulic drive apparatus, if the maximum tilting angle of the hydraulic motor is set low, the upper limit of the output torque is set low, and even if the engine accelerator pedal is depressed most on the low friction road surface, it is possible to suppress the slip of the vehicle.
According to the above-described hydraulic drive apparatus, if the minimum tilting angle of the hydraulic motor is changed, it is possible to set the upper limit for the rotation number of the hydraulic motor, and the speed upper limit of the vehicle to which the hydraulic drive apparatus is applied can be limited. According to the vehicle to which the hydraulic drive apparatus is applied, the speed upper limit can be suppressed to a low value by setting the minimum tilting angle to a large value. As a result, there is effect that even when the engine accelerator pedal is depressed most, the vehicle moves at low speed, and it becomes easy to carry out operation in a narrow place (see Patent Document 1 for example).
Patent Document 1: Japanese Patent Application Laid-open No. 2004-144254